make the bottom type immediate

Before:

    define void @_ZN3bar19h9d9b965357abc043af4v0.0E({}* noalias nocapture readnone sret, { i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone)

After:

    define void @_ZN3bar19h9d9b965357abc043af4v0.0E({ i64, %tydesc*, i8*, i8*, i8 }* nocapture readnone)
auto merge of #10107 : thestinger/rust/immediate, r=huonw